Reduce ActiveRecord connection pool from 5 to 2

When increasing concurrency by using a multi-process web server like
Unicorn, we must be aware of the number of connections our app holds to
the database and how many connections the database can accept. Each
process requires a different connection to the database. To accommodate
this, ActiveRecord provides a connection pool that can hold several
connections at a time.

We are setting the pool to 2 connections or the value specified in the
`DB_POOL` environment varriable. We are also setting `reaping_frequency`
which is available in Rails 4.

Heroku provides managed Postgres databases. Different tiered databases
have different connection limits. The Starter Tier "Dev" and "Basic"
databases are limited to 20 connections. Production Tier databases
(plans Crane and up) have higher limits.

Once our database has the maximum number of active connections, it will
no longer accept new connections. This will result in connection
timeouts from our application and will likely cause exceptions.

It is possible for a connection to get into a bad or unknown state. Due
to this, Heroku recommends setting the pool of our application to either
1 or 2 to avoid zombie connections from saturating our database.

With Unicorn, this could mean that over time a 3 worker dyno which
normally consumes 3 database connections could be holding as many as 15
connections (5 default connections per pool times 3 workers). To limit
this threat, we lower the connection pool to 2 and enable connection
reaping.

`reaping_frequency` is telling ActiveRecord to check to see if
connections are hung or dead every 10 seconds. While it is likely that
over time our application may have a few connections that hang, if
something in our code is causing hung connections, the reaper will not
be a permanent fix to the problem.

https://devcenter.heroku.com/articles/concurrency-and-database-connections